Escape Dynamics is developing external microwave propulsion technology that allows reusable single-state-to-orbit space flight. In external propulsion all or part of the energy necessary for launch is coming from the ground in the form of a focused microwave beam allowing a dramatic increase in efficiency of propulsion compared to chemical rockets and enabling engines with specific impulse above 750 seconds.

Escape Dynamics’ baseline technology uses a wireless energy transfer system based on millimeter-wave high power microwave sources. The baseline frequency is 92 GHz; however, other mm-wave frequencies (90-170GHz) are also considered. The energy is delivered to the moving vehicle via a phased array of antennas enclosed in proprietary side-lobe suppressing radomes, which ensure safety of the energy transfer.

Our baseline propulsion approach is a thermal thruster which uses hydrogen as a working fluid and a heat exchanger for coupling external microwave energy into the thermal energy of the hydrogen. External microwave energy is absorbed in a ceramic matrix composite (CMC) heat exchanger with dimensions of approximately 3meters by 5meters. The hydrogen is initially stored as a liquid in a cryogenic tank and is supplied to the heat exchanger via a turbopump designed to raise the hydrogen’s pressure to approximately 150atm. The hydrogen is heated to above 2000C as it flows through the heat exchanger and is exhausted through an aerospike nozzle optimized for a SSTO flight. The heat exchanger also serves as a primary component of the thermal protection system (TPS) during the return from orbit.

The specific impulse of 750-850 seconds is consistent with single-stage-to-orbit operation and allows for a propellant mass fraction below 72% thereby enabling a path to full and rapid reusability. Our thermal thruster approach allows useful payload fractions between approximately 8% and 12%. In comparison, chemical rockets today are limited to Isp of approximately 450 seconds, are completely or partially expandable, and typically operate with useful payload fractions of 1.5%-3%.

Escape Dynamics is also developing a next generation system, which relies on the direct heating of plasmas flowing through a resonant cavity with a weight similar to that of the weight of the heat exchanger in a thermal thruster. The goal of this development is to allow operation with specific impulse above 1,500 seconds, leading to propellant mass fraction below 50%, which is comparable to airplanes

In the baseline mission, the launch vehicle is taking off vertically from a dedicated space port equipped with a high power microwave launch array (L.A.) which powers the vehicle through the initial part of the trajectory and in which the focusing of millimeter-wave microwave energy is optimized for a 50-100km range. As the launch vehicle reaches the handoff point, the booster array (B.A.), located approximately 200-250km downrange, is activated and provides the power necessary for orbital acceleration. The booster array is optimized for a 350-450km range and accelerates the launch vehicle to orbital velocity.

The acceleration to orbital velocity will take approximately 300-400 seconds during which the payload will
experience up to 7g of acceleration with thrust-to-weight ratio at take-off around 1.25 and average acceleration throughout the trajectory of 3.5g.

For our first reusable single-stage-to-orbit launch system, we are optimizing the design to require minimal possible power on the ground with a current baseline of approximately 300-400MW of antenna output power for both launch and booster arrays. With the projected 50% efficiency of high power microwave sources, 400MW of output power would require 800MW of power taken from the electric grid. This power will be buffered at a substation at the launch site and released during approximately 300 seconds (1/12 of an hour) of powered ascent, leading to the total of approximately 65MWh of electrical energy consumed from the energy storage substation. The storage-based grid-tie in solution is a proprietary technology invented at Escape Dynamics that dramatically transforms the economics of external propulsion reducing the cost of grid tie-in from $100’s of millions of a more standard AC-DC approach to $10’s of millions per array.

Steering of the beam is accomplished via both phase-adjustment of antennas in the array and mechanical steering of the antennas. Based on in-house design and cost/performance optimization, we are currently focused on w-band high power microwave (HPM) emitters with approximately 500kW output per unit. One or several HPM emitters feed phased-controlled microwave energy into an antenna surrounded by a side-lobe suppressing radome. The techniques used in phased-controlled beam combining, although challenging, have been proven and advanced in plasma physics and fusion research with beams from multiple 1MW-class gyrotrons successfully combined. The effective diameter of each antenna is 8-12 meters and the focusing of energy is achieved through a packed phased array with several hundred elements. The booster array is designed to cover an area with effective diameter of 750-1000 meters required for efficient focusing of the microwave energy during orbital acceleration. The efficiency of the energy transfer from the antennas to the launch vehicle through the entire trajectory changes from above 45% during the take-off and initial acceleration to 12-20% during the final stages of launch.

Wireless Energy Transfer Tests

Typical system for wireless energy transfer includes high power microwave generators (e.g., EDI’s gyrotron-based system), antennas or antenna arrays, side lobe suppression radomes (SLSRs), a tracking/control system, and a receiver for harvesting the wireless energy.

The Escape Dynamics team has successfully tested power levels ranging from several kW to 100 kW using EDI’s high power microwave system. Demonstrations successfully completed in our lab include:

* Beaming of high power microwaves across 5 m in an enclosed environment and conversion of the wireless energy into thrust of a microwave-thermal thruster

* Efficient power beaming and tracking of a rapidly moving UAV with a gimbaled antenna using a low power W-band transmitter designed to mimic the beam of our high power microwave system enabling open beam testing in our lab.

The video below shows precise localization and tracking of UAVs, followed by the low power energy beaming demonstration at 92 GHz. The beaming system is configured to precisely match the beaming pattern of EDI’s 100 kW system, but with a low power beam allowing in-the-lab testing and operation. Separate tests with 100 kW beam are performed in an enclosure that houses a separate antenna and a receiver designed to convert energy into thrust.